1. Field of the Invention
The present invention relates generally to a seal assembly used to maintain a fluid seal between two relatively rotatable elements such as a shaft and housing, and more particularly to a seal assembly including a spacer used to position individual seal components during assembly and installation.
2. Description of the Related Art
Unitized seal assemblies are typically used to seal fluids between a rotating shaft supported, typically through a bearing assembly, by a housing. The seal assembly normally includes a seal element and a sleeve. The seal element includes a rigid member which supports a flexible seal lip made from rubber, some other suitable elastomeric material, or a resinous material such as polytetrafluorethylene or PTFE. In normal use, the seal element engages the housing and remains stationary during operation. The sleeve is typically a tubular member disposed about the rotating shaft and provides a dynamic sealing surface. During operation, the seal lip engages the sealing surface to form a fluid seal.
It is preferable that the components be preassembled; i.e., unitized, prior to shipment to avoid handling and contamination damage. Preassembly of the seal components also assists in final assembly and use of the seal. During final assembly; i.e., insertion at the shaft/housing interface, the seal assembly is press-fit into the housing after which the shaft is press-fit into the seal assembly. Thus, the seal assembly must absorb the axial load necessary to install the shaft.
Under proper operating conditions, it is necessary to maintain a small axial space or gap between the seal element and sleeve; i.e., a noncontacting relationship between the rigid member or seal support and an outwardly extending portion of the sleeve. Such a gap prevents generation of frictional heat during operation of the seal assembly as it is known that heat reduces seal life. As a significant axial load is applied to the seal during installation, which may result in a metal to metal or some other contacting relationship between the seal element and sleeve, it is necessary to provide a load transmitting device to transmit the installation load from the shaft to the housing. Such a device enables installation of the seal assembly and shaft while preventing a contacting relationship between the seal element and sleeve, thus eliminating frictional heat during use of the seal assembly.
In order to prevent contact between the seal element and sleeve, prior unitized seals have used elastomeric bumpers or nibs for axially spacing the seal element and sleeve. One problem with such a system is that rubber or elastomeric thrust bumpers are sometimes compressed as a result of the large axial mounting forces. While it is the intent that such elastomeric bumpers or nibs wear away during the initial phase of seal operation to provide clearance and thus minimal contact, the initial wearing away period can result in significant frictional heat which reduces the life of the seal assembly.
Some seal assembly mounting procedures require that the sleeve stop on an inner beating race. Mounting procedures of this type require tight control of axial stack up tolerances in order for the seal to function properly. When using an elastomeric bumper or nib, compression of the elastomeric bumper can result in improper seal assembly alignment. It is known to provide a seal assembly with a thrust bumper manufactured from a non-compressible material different from that used to form the seal lip. One type of such a seal assembly utilizes a thrust bumper made from a resilient, high temperature resistant, lubricous, plastic material. While such a thrust bumper overcomes the problem of compressibility and provides for axial separation of the sleeve and the seal element, the seal assembly requires an additional element which increases both the cost and manufacturing time of the seal assembly.
While the preceding approaches may have some limited use, it is desired to have a unitized seal assembly which eliminates the need for either a fixed internal thrust bumper or contact with an adjacent inner bearing race. Further, a seal assembly is desired that provides an axial gap between the sleeve and the seal element. The gap prevents contact between the seal element and the sleeve, either metal to metal or elastomeric to metal, thus eliminating frictional heat and seal degradation during initial start up and use of the seal.
Additionally, prior art seal assemblies are difficult to ship and handle; they compose two individual pieces with the seal element simply slipped over the sleeve. It is desirable to have a seal assembly which is unitized; i.e., the components are preassembled prior to shipping. Such an assembly facilitates installation while properly maintaining an axial clearance between the sleeve and the seal element.